1. Technical Field
The present invention relates to a light-emitting module.
2. Background Art
The package of an optical transmitter including a semiconductor laser element has two types as follows:
One is a so-called butterfly type package disclosed in U.S. Pat. No. 6,856,475. The other is a so-called coaxial type package disclosed in U.S. Appl. Pub. No. 2004-245446A and 2003-043868A.
The laser module described in U.S. Pat. No. 6,856,475 comprises a butterfly type package, a base accommodated in the package, a thermo-electric cooler arranged between the package and the base, and a device to be controlled in temperatures such as a laser diode, a lens or a photo diode mounted on the base. An object of the invention according to U.S. Pat. No. 6,856,475 is to provide a laser module that covers each element to be controlled in temperatures mounted on a base with a radiation-shielding member so as to reduce heat transmitted from a package to the device to be controlled in temperatures and reduce the amount of heat to be cooled by a thermo-electric cooler thus suppressing the power consumption.
On the other hand, a laser module having a coaxial type package disclosed in US 2004-245446A or US 2003-043868A must measure the temperature of a laser diode (hereinafter referred to as an LD) in order to control the LD so that a thermistor is arranged in close to the LD.
However, with a coaxial type package, a distance between an LD and a case is about 0.2 mm. Accordingly, arranging a thermistor in close to an LD inevitably positions the thermistor closer to the case wall surface. The thermistor is thus influenced by the temperature outside the package (radiation from the case wall surface) thus resulting in an error in the temperature measurement of the LD. A thermo-electric cooler that mounts the LD cools or heats the LD based on the temperature measured by the thermistor so that an error in the temperature measurement leads to excessive cooling or excessive heating. As a result, a wavelength drift occurs.
To be more specific, assuming the temperature of the LD to T[° C.], the temperature signal from the temperature measuring element includes a rise or drop in the temperature Δ[° C.] caused by the radiation from the case. Thus, the LD is excessively cooled or heated to T-Δ[° C.] by the thermoelectric cooler. Thus, it is difficult to accurately control the temperature of the LD to a predetermined value.
In particular, this problem is critical in a case, when an optical transmitter is used for optical communications of DWDM (Dense Wavelength Division Multiplexing). The wavelength interval of signal light is set to be quite narrow in optical communications of DWDM. Thus, it is necessary to extremely minimize the variation in the light-emitting wavelength of the LD whose emitting wavelength varies with temperature. In a conventional optical transmitter, for example, a circuit is provided for maintaining the emitting wavelength of the LD constant to suppress the fluctuation of the emitting wavelength. However, this approach results in an increased size of a peripheral circuit of the optical transmitter.
A heat-shielding member described in US 2003-043868A covers the LD and other elements to be cooled at the same time although it is difficult to provide such members within a coaxial type package. More specifically, the coaxial type package has a structure where a case supports a lens so that the spacing between the LD and the case is extremely small, accordingly, it is difficult to provide a heat-shielding member described in U.S. Pat. No. 6,856,475.